Long chain fatty acid esters derived from renewable oils via transesterification are known as biodiesel, which is a new kind of pollutionless, recoverable energy source. Biodiesel has combustion properties very similar to those of a petroleum system. Since it significantly reduces toxic emissions by 50% when burned as a fuel, great interest has been raised for the study and application of biodiesel. (Hideki Fukuda et al., Biodiesel fuel production by transesterification of oils, J. Biosci. Bioeng., 2001, 92(5):405-416).
Currently, biodiesel is manufactured mainly through chemical methods. More particularly, vegetable oils and/or animal fats are used as a source of long chain fatty acids and a transesterification reaction between the long chain fatty acids and some low carbon alcohols, such as methanol or ethanol, is carried out in the presence of an acid or base catalyst, and methyl ester or ethyl ester of the fatty acids are obtained. However, some inevitable disadvantages exist in chemical methods as follows: free fatty acids and water in the renewable oils raw material severely spoil the reaction; emulsion is undesirably formed due to the poor solubility of alcohol in renewable oils, and the following processes are complicated accordingly; the need for significantly excess alcohol results in greater energy consumption in later distillation and reflux processes of the alcohol.
In contrast, synthesizing biodiesel through bio-methods has the following advantages: mild reaction conditions, non-toxic emissions, and enzyme-catalyzed reactions are not affected by free fatty acid and small amounts of water in the renewable oils used as raw materials. Therefore, bio-methods are consistent with the requirement of developing Green Chemistry, and thus have attracted more and more attention. (Yomi Watanabe et al., Continuous production of biodiesel fuel from vegetable oil using immobilized Candida antarctica lipase, JAOCS, 2000, 7(4): 355-358). In previously reported processes of preparing biodiesel through biological enzyme-catalyzed methods, low carbon alcohols such as methanol and ethanol are mainly used as acyl acceptors in the reactions. However, problems exist in these processes where low carbon alcohols are used as acyl acceptors. For example, low carbon alcohols are harmful to enzyme molecules and shorten the functional life of enzyme. Moreover glycerine is generated as a by-product, which tends to block the active site of the immobilized enzyme and thus is harmful to enzyme activity. (Yuji Shimada et al. Enzymatic alcoholysis for biodiesel fuel production and application of the reaction to oil processing, Journal of Molecular Catalysis B: Enzymatic, 2002, 17: 133-142).